#Python 2, 52 bytes

    a=b=input()
    while len(set(str(a)))!=1:a+=1
    print a-b

Python 2 has several tricks that make this shorter. For example, input is numeric, so we don't need to cast to int. (-5 bytes) We also don't need to put parenthesis around the `a-b` (-1 byte)

Use this script to verify all test cases:

    def f(i):
        a=b=i
        while len(set(str(a)))!=1:a+=1
        return a-b
    
    inputs = [8, 100, 113, 87654321, 42, 20000, 11132]
    outputs = [0, 11, 109, 1234567, 2, 2222, 11090]
    
    for i in range(len(inputs)):
        print(f(inputs[i]) == outputs[i])

You may also [try it online!](https://ideone.com/7ZVxZq)